Amplifier circuits



K. WILH ELM AMPLIFIER CIRCUITS Oct. 1, 1940.

Filed Feb. 24; 1939- INVENTOR. KARL jLI-IELM ATTORNEY.

70 SOURCE OFS/GNALS Al 3 55 E w Patented Oct. 1, 1940 AIVIPLIFIER CIRCUITS Karl Wilhelm, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. 11., Berlin, Germany, a corporation of Germany Application February 24, 1939, Serial No. 258,138 In Germany April 29, 1938 '7 Claims.

In the case of transformer coupling in audio amplifiers the lower limit frequency is determined by the primary inductance of the transformer and by the inner resistance of the preceding tube. At the lower limit frequency the volume drops to 0.7 of the amplification existing at the intermediate audio frequencies. The volume is further reduced at the low frequencies on account of the lesser effect of the sounding board of the loudspeaker. It is known to achieve a raising of the low audio frequencies by resonance between the primary inductance of the transformer and a condenser placed between the anode and the transformer. The disadvantage of this circuit resides in the fact that the amplification decreases unless the plate potential is very high, because the resistance across which the direct plate current is applied can be chosen only so high that the direct plate potential at the tube and the direct plate current are still sufiiciently high; otherwise excessive control appears and the inner resistance becomes so high that no resonant rise of voltage takes place. When applying the invention, resonance at low audio frequencies is achieved without a reduction of the amplification.

The present invention, therefore, relates to an audio' amplifier stage with a transformer in the plate circuit of a tube and consists in that from a voltage divider, having the alternating plate potential and consisting at least of a reactive impedance and ohmic resistance, an alternating potential is applied to a grid of the tube, more especially to the positively biased screen grid.

The alternating potential has such a phase displacement and such a value that atalow frequency of the frequency-range to be'transmitted, resonance appears between the primary inductance of the transformer and the capacitive component of the inner resistance of the tube.

The control with the potential which is displaced in phase may be carried out, for instance, at the control grid whereat the potential to be amplified and the potential which is displaced in phase are connected in series. However, it will be more suitable to apply the potential which is displaced in phase to a special grid, for instance to a distribution grid, screen grid or protection grid. In this case in place of a single grid tube such as used in the resonance circuit, a multigrid tube is employed.

Fig. 1 shows one embodiment of the invention; Fig. 2 an analysis thereof; Fig. 3 shows the electrical equivalent diagram of Fig. 1; Fig. 4

shows a simplification of the diagram of Fig. 3;

Fig. 5 graphically shows the operation of the invention in Fig. 1; Fig. 6 illustrates a modified form of the invention.

In Fig. 1 there is shown a screen grid tube (pentode) I having a transformer T in the plate 5" circuit thereof, the tube acting as a voltage amplifier tube. The voltage divider employed in accordance with the invention for controlling the screen grid potential consists of a condenser C and resistor R1. across which at'the same-time 1d the screen grid direct current potential is applied. The following values may be used in this circuit:

C=20,000 micro-microf'arads (mmfi) R1=50,000 ohms L1=100 henrys It should be noted that the resistor R1 has placed in parallel thereto the inner resistance of the cathode-screen grid path of the tube amounting in the said tube to about 50,000 ohms. This in- 2 0 ner resistance'is determined, for instance," from the through-grip (reciprocal of amplification factor) and the steepness of the cathode-control grid screen grid system of the tube. The through-grip can be adopted as about 3% and 25 the steepness appertaining to the controlgrid as about 0.5 mA. per volt relative to the screen grid.

Fig. 2 shows the vector diagram for ascertaining the inner resistance. The diagram shows the 3g alternating plate potential Ua;'the potential m; at the capacity C; the potential us at the resistor R1, and the plate current Ja. It is seen that the alternating plate potential Ua is divided up by means of the voltage divider into two poten- 35 tials extending at right angles to one another. The alternating screen grid potential us causes an alternating plate current J9. of the same phase, which lags behind the alternating plate potential Ua by about Consequently, the inner resistance of the tube, aside from the ohmic component, also has a substantially capacitive component. This is known as such from the socalled tuning regulation tubes employed in receivers with automatic sharp tuning (AFC).

The equivalent circuit is shown in Fig. 3 in which R1 represents the ohmic part of the inner resistance, and C1 represents the capacitive part of the, inner resistance. C is the condenser as shown in Fig. 1, and R is the'parallel connec- ,50 tion of R1 in Fig. 1 to the said inner resistance of the cathode-screen grid path. This equivalent circuit can be simplified in accordancewith Fig. 4 in combining the capacities and the ohmic resistances. The potential E of the voltage 55 source then has a value different from E in Fig. 3. The capacity C' in the hitherto given example is calculated to about 40,000 mmf., and the resistance R is found to be about 10,000 ohms. The condenser C will be in resonance with the inductance L1 at about 70 cycles. Since the resistor R is just as high as in the case of a single grid tube, a resonant rise of voltage takes place though a pentode is employed.

Fig. 5, representing the dependence of the amplitude L on the frequency 7, shows the way in which the circuit according to the invention operates (full line characteristic). The calculation of the ohmic and capacitive part of the inner resistance of the tube is as follows: The alternating plate current Ja is equal to the product of the alternating screen grid potential in the steepness SAs of the screen grid, if the steepness of the screen grid is meant to represent the dependence of the alternating plate current on the alternating screen grid potential:

o a[ R 1 1 118 The inner resistance is equal to the alternating plate potential divided by the alternating plate current:

1 i I" R S 'QC) The ohmic part of the inner resistance, therefore, is equal to SAS and, therefore, equal to the alternating screen grid potential divided by the alternating plate current. This is the inner resistance which the tube would have when the screen grid is connected with the anode, which is readily understandable when considering that at high frequencies the capacity C practically acts as shortcircuit connection between the anode and the screen grid. At high frequencies the capacitive part of the inner resistance is an impedance overshadowed by the ohmic part, because at high frequencies the resistance of a capacity becomes lower.

.The said steepness SAS of the screen grid can be calculated when taking into consideration that this steepness SAS which when viewed from the screen grid is smaller than the steepness appertaining to the control grid by the value of the through-grip of the screen grid through the control grid. The steepness SAS -.of the screen grid, therefore, will be found as the product of the through-grip of the screen grid through the control grid (for instance 3%) in the steepness (for instance 1.5 mA. per volt) appertaining to the control grid.

The potential, displaced in phase, instead of being applied to the screen grid from the com,- mon point of the condenser C and resistor R1, as in the case of Fig. 1, may also be derived from a'tap point on the resistor B1. In this manner it is possible to bring the ohmic part of the inner resistance upon a value difierent from that corresponding to the through-grip of the screen 'grid.

Finally, Fig. 6 shows still another mode of construction accordingto the invention. The voltage divider in this case consists of an ohmic same manner as the'voltage divider in Fig. 1.

What is claimed is:

1. In an audio amplifier network of the type comprising a tube provided with at least a cathode, signal grid, screen grid and output electrode, a source audio signal voltage coupled to the cathode and signal grid, an audio output transformer, said output electrode being connected to one end of the transformer primary winding, a series path of a reactance element and a resistance element in shunt with said primary winding, connections from said primary winding and series path to points of positive potential, said screen grid being connected to a point on said series path such that it assumes a positive potential, and said primary winding resonating the capacitive component of the cathode to screen grid impedance at the low frequency end of the audio range.

2. In an audio amplifier network of the type comprising a tube provided with at least a cathode, signal grid, screen grid and output electrode, a source audio signal voltage coupled to the cathode and signal grid, an audio output transformer, said output electrode being connected to one end of the transformer primary winding, a series path of a reactance element and a resistance element in shunt with said primary winding, connections from said primary winding and series path to points of positive potential, said screen grid being connected to a point on said'series path such that it assumes a positive potential, and said primary winding resonating the capacitive component of the cathode to screen grid impedance at the low frequency end of the audio range, said reactance element consisting of a condenser, and said screen grid being connected to the junction of the condenser and resistance element.

3. In an audio amplifier network of the type comprising a tube provided with at least a cathode, signal grid, screen grid and output electrode, a source audio signal voltage coupled to the cathode and signal grid, an audio output transformer, said output electrode being connected to one end of the transformer primary winding, a series path of a reactance element and a resistance element in shunt with said primary winding, connections from said primary winding and series path to points of positive potential, said screen grid being connected to a point on said series path such that it assumes a positive potential, and said primary winding resonating the capacitive component of the cathode to screen grid impedance at the low frequency end of the audio range and at a frequency of the order of '70 cycles.

4. In combination with an amplifier tube of the pentode type, a source of signals of a wide frequency range coupled to the tube input electrodes, an inductive load element coupled to the output electrodes of the tube which causes a decrease in signal voltage amplitude at the low frequency end of said range, a reactive impedance path in shunt with said inductive load for developing alternating current voltage, means applying said latter voltage to the screen grid of saidtube in such phase that the cathode to screen grid capacitive impedance of said tube is sufliciently high to resonate said inductive load at said low frequency end of the range thereby to compensate for said decrease.

5. In combination with an amplifier tube of the pentode type, a source of signals of a wide frequency range coupled to the tube input electrodes, an inductive load element coupled to the output electrodes of the tube which causes a decrease in signal voltage amplitude at the low frequency end of said range, a reactive impedance path, consisting of a condenser in series with a resistor, in shunt with said inductive load for developing alternating current voltage, means applying said latter voltage to the screen id of said tube in such phase that the cathode to screen grid capacitive impedance of said tube is sufficiently high to resonate said inductive load at said low frequency end of the range thereby to compensate for said decrease.

6. In combination with an amplifier tube of the pentode type, a source of signals of a Wide frequency range coupled to the tube input electrodes, an inductive load element coupled tothe output electrodes of the tube which causes a decrease in signal voltage amplitude at the low frequency end of said range, a reactive impedance path in shunt with said inductive load for developing alternating current voltage, means applying said latter voltage to the screen grid of said tube in such phase that the cathode to screen grid capacitive impedance of said tube is sufiiciently high to resonate said inductive load at said low frequency end of the range thereby to compensate for said decrease, said frequency range ,being the audio range, and said resonant point being at approximately 70 cycles. 7. In a wide frequency band signal amplifier, a tube provided with a cathode, signal grid, plate and an auxiliary electrode between said cathode and plate, means adapted to apply signals to said signal grid, a load element connected to the plate which has an essentially inductive reactance at the low frequency end of said frequency band thereby causing a reduction in signal voltage amplitude; the improvement which comprises a reactive path shunted across said load element and which develops a signal voltage thereacross, means applying said signal voltage to said auxiliary electrode in a phase such that the tube impedance existing between cathode and said auxiliary electrode has a substantial capacitive component, and said component and inductive reactance cooperating to minimize said amplitude reduction.

- KARL WILI-IELM. 

